Study On Reliable LTE Handover Technology In Rail Transit Systems

With the rapid development of urban rail transit system, the train speed becomes higher and the operation environment is more complex. In order to achieve higher capacity and better performance in high speed environment, a more stable and reliable technology is needed for train-ground wireless communication system. Long Term Evolution (LTE) system, as the next generation broadband mobile communication system, uses flattened IP network structure, and is able to provide up to 100 Mbps downlink and 50 Mbps uplink peak rate with shorter control plane delay and user plane delay. Compared with the wireless communication technology applied in urban rail transit systems, such as Wireless Local Area Networks (WLANs), LTE can provide a safer, more efficient and more reliable broadband communication services to ensure the safe operation of rail vehicles. In LTE system for urban rail transit systems, due to the high movement speed of trains and limited radio coverage of cells, cross-cell handover technology is very important to maintain the continuity and reliability of the train-ground communication and ensure communication service quality. It is necessary to study handover technology in LTE system.This thesis aims to put forward a more reliable and efficient handover strategy to improve the performance of train-ground communication system.This thesis focuses on the handover technology in urban rail transit environment. We use multi-point collaborative transmission technology to improve the existing handover mechanism. Moreover, the train location and speed are taken into account to optimize the handover performance. Simulation results show that the proposed strategy can siginificantly reduce the outrage probability and decrease the number of unnecessary handover. The the reliability of communication system is improved using our proposed stategy.The specific research contents of the thesis are as follows.(1) The thesis analyzes the basic structure of LTE system andthe details of handover process. The commom classification of handover is introduced, followed by the reasons of handover triggering. The thesis also provides a detailed analysis of the handover process and signaling process. The calculation process of handover outrage time is analysed at last.(2) The thesis studies the characteristics of rail transit environment, which have great influence of handover decision. A detailed analysis of factors which affect the area coverage is given. RRU scheme and overlapping coverage distance are discussed here. The thesis also studies a suitable wireless fading model for rail transit environment.(3) In this thesis, a CoMP based handover scheme is proposed to improve the performance of hard handover in existing LTE mechanism. Through collaboration transmission with adjacent base station, the traditional handover scheme which switchs between cells is replaced by updating collaboration sets, which can fundamentally solve the interruption problems in traditional "break before make" hard handover. The train speed for real-time is adopted to adjust the collaborations model. In addition, the train position information is also taken into consideration to improve the handover performance.(4) The thesis verifies the optimization of the proposed handover strategy. A simulation model is confirmed to analyze the handover performance with suitable simulation parameters. The perfomance of traditional handover mechanism and proposed handover strategy are compared.The simulation results show that, compared with the existing handover strategy, the proposed strategy can effectively reduce the communication outrage probability and the ping-pong handover times, which help improve the successful handover rate. The proposed method is more suitable for high speed rail transit environment and is able to ensure the quality of LTE communication services.